The major goal of this proposal is to continue to characterize nuclear binding of the estrogen receptor and to relate binding to estrogen induced responses. Recently we have described a new kind of estrogen binding site (Type II) which is present in the cytoplasmic and nuclear compartment of estrogen sensitive cells. Type II sites are of importance for two reasons: (1) They interfere with the accurate measurement of the estrogen receptor (Type I) and (2) They are stimulated by estradiol exposure and correlate well with uterine growth. Therefore, they may be of major biological importance in the mechanism by which estrogen regulates growth. Cytoplasmic and nuclear Type II sites will be characterized and their relationship to the estrogen receptor will be defined. Quantitative and qualitative relationships will be examined under various conditions and valid methods for the assay of both sites will be established. The localization of both types of sites will be studied and their physiological significance will be examined. The effects of various estrogens and estrogen antagonists will be investigated and correlated with the various uterotropic responses (RNA polymerase activity, RNA polymerase initiation sites, protein and DNA synthesis and uterine hypertrophy and hyperplasia. A system for the study of the effect of estrogens on specific cell types will be developed and characterized. An in vitro model system for the study of nuclear binding, nuclear processing and cytoplasmic replenishment of Type I and II sites will also be investigated.